As magnetic recording media for audio or video equipment, or computers, etc., general usage is made of magnetic recording media comprising a nonmagnetic support having thereon a magnetic layer comprising a binder and a ferromagnetic powder dispersed therein.
These magnetic recording media are typically produced by coating on a nonmagnetic support a magnetic coating solution. This magnetic coating solution is prepared by dispersing a binder component, such as a resin component, and a granular component, such as a ferromagnetic powder, in a solvent, to thereby form a magnetic coating solution, and after application of a treatment such as a magnetic field orientation treatment, then performing a drying treatment and/or a surface smoothening treatment, and cutting the coated support into the desired form.
Although it is generally considered that the granular component is firmly fixed onto the magnetic layer in the surface of the magnetic layer as produced above, and that the surface is very smooth, it is nonetheless appreciated that, in actual practice, the granular component, such as ferromagnetic powder, is fixed insufficiently on the surface of the magnetic layer. Such an insufficiently fixed granular component is released during running and attaches onto a magnetic head, sometimes causing clogging of the magnetic head. Moreover, in the case of a video tape, for example, dropout is caused sometimes. With such a release of the ferromagnetic powder, the amount of the ferromagnetic powder existing in the vicinity of the surface of the magnetic layer is correspondingly decreased and, thus, with repeating of the running, a problem arises in that electromagnetic characteristics are reduced and, as a result, the output is decreased.
In order to eliminate such problems of dropout, the clogging, and the decrease of the output, JP-A-62-172532 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") proposes a method in which the surface of the magnetic layer is subjected to a grinding treatment.
That is, in accordance with the above grinding method, the surface of the magnetic layer which has been made smooth is ground by the use of a high hardness grinding device such as a diamond wheel or a fixed sapphire blade to thereby remove the granular component in the state that is easily released, or materials attaching onto the surface of the magnetic layer, which, as a result, decreases the amount of the material released from the surface of the magnetic layer.
In view of the above background, various investigations have been made for determining methods of decreasing the amount of the material being released, and, in this regard, effective methods other than the grinding of the magnetic layer of the magnetic recording medium have been proposed. For instance, a method in which the magnetic layer of the magnetic recording medium is abraded with an abrasive tape has been proposed in U.S. Pat. No. 5,009,929. It has been found that this method decreases the dropout and the clogging, and permits the production of a magnetic recording medium having good running durability. In accordance with the method, however, the abrasive in the surface of the magnetic layer is exposed, thereby increasing abrasive ability and, as a consequence, there arises a problem that the head is damaged.
As another approach, JP-A-61-57036 proposes a magnetic recording medium in which abrasive particles having Mohs' hardness of at least 6 are incorporated in a proportion of 3 to 15% by weight based on the weight of the magnetic powder, and the average number of the abrasive particles per unit area in the surface of the magnetic layer is controlled to at least 0.25/.mu.m.sup.2.
In accordance with the above method, discoloration due to baking of the head and the clogging are eliminated, and there can be obtained a magnetic recording medium having a high output. In order to control the average number of the abrasive particles per unit area of the surface of the magnetic layer to at least 0.25/.mu.m.sup.2, there is employed a method in which the ferromagnetic powder and the binder are dispersed over a period of 4 hours, and after addition of .alpha.-Al.sub.2 O.sub.3 particles, the resulting mixture is dispersed for 4 additional hours. That is, in the course of the dispersing process, the abrasive is added to decrease the degree of dispersion and to weaken the affinity to the binder, thereby facilitating migration of the particles to the surface of the magnetic layer. In this method, however, the abrasive is enclosed by the binder insufficiently.
Thus, the dispersibility of the abrasive in the surface of the magnetic layer is insufficient and, therefore, irregularities in the surface are formed, leading to an increase in the noise and a reduction in the C/N. Moreover, since the amount of the abrasive is relatively large in the surface of the magnetic layer, the filling degree of the ferromagnetic powder is decreased and the RF output is decreased.
Also, JP-A-62-16242, JP-A-62-16244 and JP-A-62-16245 disclose separate dispersion of the abrasive. This separate dispersion improves surface properties and the filling degree. However, the application of the separate dispersion alone causes the disadvantage that since the surface of the abrasive is covered with the binder, the abrasion ability is reduced, leading to a decrease of the output and, furthermore, since the surface properties are improved, the .mu. value is increased.
In addition, JP-A-63-259830 discloses an abrasive tape treatment. In accordance with this treatment, the abrasive ability is increased, and, therefore, the decrease of the output can be improved and the increase of dropout DO can be prevented. However, since a separate dispersion of the abrasive is not applied, the dispersibility of the abrasive is reduced, the irregularities in the surface are increased, and the RF output and C/N are decreased.